The soybean is a highly nutritious food and in much demand as a health food or for other purposes. Therefore, soybeans are used as ingredients for various kinds of foods, such as “tofu” (bean curd), “miso” (bean paste), “shouyu” (soy sauce) and snack foods. To improve the production efficiency of such soybean-based foods, soybean powders are industrially used. Unfortunately, soybean powders inherently have an unpleasant, grassy odor, called a “soybean odor.” This odor is attributable to hexanol, hexanal and other odor components produced by lipoxygenase, a fatty-acid oxidation enzyme residing in the beans. However, in recent years, soybean powders free from this soybean odor are available. Such odorless soybeans powders are produced by including a deodorizing treatment in the powder production process.
Among the nutrients contained in soybeans, isoflavone is known for its particularly high nutritious value. The demand for soybean-embryos, as well as whole soybeans, is increasing since soybean-embryos have a high isoflavone content. However, soybean-embryos also contain lipoxygenase. Therefore, as in the case of whole soybeans, a deodorizing treatment is necessary to industrially use the soybean-embryos.
One conventional method for deodorizing soybeans or soybean-embryos includes deactivating lipoxygenase with a chemical additive. Another method uses a new soybean from which lipoxygenase is genetically removed. However, out of concerns for safety and other factors, hot steam is most commonly used to deactivate lipoxygenase (refer to Patent Document 1).
A technique developed for the aforementioned conventional deodorizing treatment using hot steam employs a closed container with an internal screw conveyer; the soybeans, soybean-embryos or similar foodstuff are put into the container and deodorized by introducing hot steam into the container while conveying them with the screw conveyer (refer to Patent Documents 1 and 2). In this conventional method, a drying process using a dryer, which is typically a hot-air dryer, is carried out after the deodorizing treatment using the screw conveyer.
The conventional soybeans/soybean-embryos processing machine performing the aforementioned treatments has problems in the deodorizing process and in the subsequent drying process. In the deodorizing process, too high a treatment temperature or too long a treatment time will cause thermal denaturation or elution of proteins or other nutrients contained in the soybeans/soybean-embryos, while too low a temperature will lead to inadequate deodorization. Thus, proper control of the temperature and time is particularly important in the deodorizing process. Unfortunately, in the aforementioned method using the screw conveyer, the temperature in the closed container easily changes, which makes the treatment conditions unstable and thereby causes the finally obtained soybean powder to be inconsistent in quality. Another drawback of the conventional method is that the method requires separate systems for the deodorizing and drying processes; this configuration makes the production process complex, increases the production cost and elongates the processing time.
Still another problem of the deodorizing process is its poor drying efficiency: In the deodorizing process, the moisture content rises to a range from 10 to 18% since steam is used to deactivate and sterilize the soybeans. By contrast, to crush the soybeans into fine powder, it is necessary to lower their moisture content to a range of 7 to 8%. Drying the soybeans to such a low level after the aforementioned deodorizing treatment requires a considerable length of time; the drying time can be as long as 20 hours.
Patent Document 1: Japanese Unexamined Patent Application Publication No. S60-083556
Patent Document 2: Japanese Unexamined Patent Application Publication No. S60-114159
Patent Document 3: Japanese Unexamined Patent Application Publication No. S62-003753